1. Technical Field
This invention concerns a process for the recycling of cross-linked polymeric materials, in particular coming from scraps of electric cable coating materials and from unused electric cable coating materials.
2. Discussion of Related Art
The wide use of cross-linked polymers, and in particular cross-linked polyethylenes, for the coating and insulation of electric cables is known; cross-linked, i.e. a three-dimensional cross-link of intermolecular bonds allows, in fact, favourable mechanical properties to be achieved for this application and in particular, it makes such materials substantially infusible (as a result, their hot mechanical collapse is limited and their thermal stability is improved).
It is also known that the processes for coating electric cables with cross-linked polymeric materials, mainly those obtained by extrusion, cause scraps: such scraps just appear as formed by cross-linked materials to a varying cross-linking degree and generally, with different properties and compositions. In fact, the composition and nature of these waste materials besides depending on the type of the basic polymer, also depend on the cross-linking method being used, which determines the cross-linking degree as well as the quantity and kind of residues within the material (specifically, these materials may contain variable quantities of silanic agents, peroxides and various catalysts).
Moreover, in many instances and especially when the cross-linking process is carried out with the help of silanic agents, the cross-linking degree of the scraps from extrusion also depends on the stocking conditions of the scraps themselves, specifically on the presence of water or air humidity.
After all, both the cross-linking degree and composition of cross-linked polymer scraps generated during a manufacturing cycle of electric cables are generally highly variable, save the fact that anyway said materials cannot be regarded as thermoplastic materials.
In fact, cross-linking makes the polymeric materials substantially infusible and, if this may represent an advantage from an applicative standpoint, it does anyway limit recovery and recycling opportunities of such materials: therefore, known recycling processes of polymeric materials are applicable to thermoplastic materials only, which once brought into their melt state, they may be reworked according to common technologies, usually mixed with a virgin polymer of same nature or compatible with it.
At present, however, scraps of cross-linked polymeric materials are not recycled being regarded as non recyclable materials and therefore they are systematically eliminated by burying in dumps or by burning: obviously, such solutions are not satisfactory both from a costing viewpoint--as costs increase considerably with the gradual decrease of dumps availability--and for their impact on the environment. It is obvious that these solutions apply equally to both the cross-linked polymer scraps coming from the manufacturing processes of electric cables coatings and to the coatings themselves made of cross-linked polymeric material, whenever the cables are withdrawn and/or replaced at the end of their service life.
In order to solve the problem of recovering cross-linked polymeric materials, it is customary to carry out a mechanical degradation thereof to reduce them to fine powder utilizable as a filler mixed with virgin polymers.
For instance, patent JP 04-197456 discloses that cross-linked material is heated and submitted to extremely high mechanical shear stresses in a Bambury-type internal mixer: a very fine (0 to 500 .mu.m) powder is obtained by the end of the process lasting 10 to 60 minutes. A similar process is also disclosed in patent JP 57-136 wherein degradation of cross-linked material to its powder state is carried out in a single-screw extruder or in a cylinder mixer or still in a Brabender-type mixer.
In both processes cited above, the cross-linked polymer is reduced to a powder that, anyway, may not be used directly alone due to its lack of cohesion, but it should be used as an additive to a virgin polymer; additionally, this powder has the drawback of a very low apparent density, hence being very bulky.